The present invention relates to a coupling device for transmitting torque between a first part and a second part, which are rotatable relative to one another. In general, such a coupling device comprises a multi-plate coupling unit having axially and alternatingly arranged first plates and second plates. The first plates are each connected in a rotationally fixed manner to either the first part or the second part. The second plates are each connected in a rotationally fixed manner to the other of the first part or the second part. The coupling device further comprises an actuating assembly for axially loading the multi-plate coupling unit by means of a setting piston arranged opposite the multi-plate coupling unit. The actuating assembly includes a first annular chamber filled with a highly viscous fluid and delimited by the setting piston. A rotational member is arranged in the first annular chamber. The first annular chamber of the actuating assembly is formed in either the first part or the second part. The rotational member is connected in a rotationally fixed manner to the other of the first part and the second part. Relative rotation between the first part and the second part causes pressure-loading and displacement of the setting piston towards the multi-plate coupling unit to close the unit as a result of the shear of the highly viscous fluid in the annular chamber and thereby transmitting torque between the first part and the second part.
Coupling devices of the type described above are offered by the Applicant under the name of Visco-Lok coupling and were described for the first time in German Patent Application P 43 27 519.2 and German Patent Application P 43 43 307.3. To balance the volume within the first annular chamber under conditions where the setting piston is pressure-loaded, a reservoir is provided in the first or second part, whichever part contains the first annular chamber. The reservoir is formed by a second annular chamber and a pre-tensioned annular piston. Within the first annular chamber the rotational member covers an annular groove whose circumferential length is delimited and whose first end is connected to the reservoir and whose second end is connected to the portion of the first annular chamber which directly adjoins the setting piston. There can be provided means upon when the relative directions of rotation of the first part and the second part are reversed it also causes a reversal of the connections of the ends of the annular groove with the reservoir and, respectively, with the portion of the first annular chamber which directly adjoins the setting piston.
The actuating assembly of the Visco-Lok coupling of the above-described type is provided with an enormously high pressure capacity, but it has a limited conveying volume. A first disadvantage of this coupling device is that the reaction speed is restricted because prior to closing the multi-plate coupling unit the play in the coupling unit has to be overcome at the relatively slow setting speed of the setting piston. A second disadvantage is that the play increases due to wear of the coupling device, which has a pronounced effect on the reaction time of the multi-plate coupling unit.
In view of an ABS compatibility of a vehicle with a hang-on configuration, whose driveline is provided with a Visco-Lok coupling, the delayed reaction behavior is often desirable. However, in other cases, a short reaction time is necessary. Regardless of the desired reaction time, it is disadvantageous if the reaction time is substantially influenced by the wear condition of the plates in the coupling unit and by the installation tolerances.
It is therefore the object of the present invention to provide a means that ensures a uniform and constant reaction of the coupling device of the described type, independently of the wear condition of the plates and the installation tolerances.